Conventionally, digital speakers are being developed which reproduce a digital audio signal not by converting it to an analog signal but directly supplying it to a speaker (e.g., see Patent Literature 1). The digital speaker described in Patent Literature 1 assigns weights to a plurality of voice coils wound around a voice coil bobbin respectively so that a drive force corresponding to each bit of the digital signal is generated, the polarity of a certain voltage applied to each voice coil is changed according to the binary value of the respective two bits of the digital signal and the direction of a current flowing through the voice coil is thereby set according to the binary value. This configuration allows a drive force to be generated at a ratio corresponding to quantization of the digital signal.
Furthermore, speaker units are being proposed which apply a digital/analog conversion apparatus that generates an analog signal of high quality from a digital signal to a drive apparatus of a digital speaker to thereby improve quality of reproduced sound and realize circuit scale reduction (e.g., see Patent Literature 2). The speaker unit described in Patent Literature 2 converts an n-bit output of a delta-sigma modulator to a thermometer code through a formatter, performs mismatch shaping processing using a post filter, inputs the output to a buffer circuit, controls a coil with the digital signal outputted from the buffer circuit and adds a magnetic field thereto (see paragraphs 0063 and 0078).
On the other hand, vibration plates of speakers used for mobile devices such as acoustic devices, video equipment and mobile phones are required to have the ability to accurately reproduce clear sound in a wide frequency band, and a high frequency range in particular. Therefore, the material of the vibration plate is required to have a high elastic modulus so as to give sufficient rigidity to the vibration plate and a low density so as to reduce the weight of the vibration plate, which are apparently mutually contradictory characteristics. Especially vibration plates for digital speakers which are becoming a focus of attention in recent years are strongly required to satisfy these characteristics from the standpoint of requirements for vibration response.